Electric power grids contain many devices that can restrict current flow. For example, such devices can include transformers, underground cables, overhead power lines, connectors, circuit breaker, and switch contacts, electric motors, and/or generators. The impedance of a device, defined by a ratio between voltage and current and measured in the units of ohm (Ω), can characterize an amount of current flow restriction. A mathematical inverse of an impedance can be an admittance, defined by a ratio between current and voltage and measured in the units of mho (1/Ω). In some single-phase alternating current (AC) power systems there are two conductors such as two-phase conductors or one phase conductor and one neutral conductor. The conductors can connect electrical components of the power system. Depending on how the electrical components are connected in a three-phase AC power system, the power system can be operated as a 3-wire three-phase system without a neutral conductor or a 4-wire three-phase system with a common neutral conductor.
In addition, AC voltage and current may be amplified or reduced by means of a transformer or a power electronic device functions like a static VAR compensator. With power flow being the product of voltage and current, amplification of the voltage can reduce the current to enable the same power flow as done in a transformer. The gain of a transformer, defined by a ratio between input current and output current or the ratio between input voltage and output voltage is typically unitless. Current flow reduction can lead to less power loss when transmitting power through a power line. Transformers are therefore essential components in AC power transmission and distribution circuits.
Impedance, admittance or gain of electrical components in a power grid may be characterized by a representative set of parameters that may consist of complex valued numbers. Deviations in the complex valued numbers representing impedance, admittance, gain, or a deviation in the filtered rate of change of the characteristic value of electrical components may indicate abnormalities, changes or nonconformities of the power system.